I am just wondering why my Mokal Mirage makes a brrrup sound? Almost like the sound when your running out of air/co2 but shorter. And the next shot is perfect. I am thinking the ball is not in proper position and that interferes with the hammer hit. I have made a new extended sear and grip frame for this custom gun (major surgery). It moves the pistol grip/trigger group forward about 2".
In fires without a problem most of the time, 20% of the time burrp.

It also seems to do this when fired without paint. So I may be a sear / bolt match-up problem.

Here's a sear similar to the one in the gun. The actual custom sear pivots near the middle. The length of the sear could be the problem. Leverage could be working against pushing up on the sear to block the hammer back,

One other thing to check, from my experience working at a field that rented Mirages, see if the valve seal has come unseated from the valve cap. At least on the later model ones we rented the valve seal was pressed into the valve cap and the center crimped in to hold it. Sometimes the seal would wiggle out and then all kinds of weird things would happen. Generally low velocity and leaks, but burping was also sometimes a symptom.